It is now a pretty much meaningless term. Although it has contributed some of the most hilarious terms to the game's lexicon, eg Mongolian Earthworm Cleave.

Yeah, the usage of the word "cleave" in arena comp names has nothing to do with its actual meaning in English and very little to do with its conventional use in the rest of WoW (a "cleave" attack being one that does significant "splash" damage to a limited number of opponents near the primary target).

It's sort of like how the Japanese use English words as linguistic decoration with little respect for their actual meaning. Banana cleave! Brimstone cleave! Fake hair cleave! Clamato cleave!

World of Warcraft started life as a Computer Roleplaying Game, where part of the fun of the game experience was pretending to be your character. Stuff like applying poisons and eating food enhanced the verisimilitude of the experience of playing a fantasy character in another world. Now that game has changed to become a tactical arcade lobby game.

A lot of this lingo is from IRC or earlier online/chat networks (like CompuServe and GEnie). Then it crossed over to SMS (Text Messaging) and teens devolved it into LOLspeak. It really isn't WoW specific.

Most arena related things I don't get, but I don't try to either, since I don't bother with arena.

Also I hate reading guides for a class I am not that familiar with, only to be assaulted by a billion abbreviations that I need to look up. It is just annoying, especially since there's often a couple different versions for several things.

Early on I was confused by people using "bio" for bathroom break, mostly because it as always Swedish people who used this term in my experience. And bio is often used for "movie theater", it's silly but oh well. It's a long time ago :P

True confession:
For the longest time, I was oblivious to what the heck a faceroll was.

When I figured it out, not only did I literally LOL, but that became my default mental image of certain in-game antics and the people responsible for them. Huntards, failocks, retardins (and given I have a hunter, lock, and retpally I learned most of what NOT to do from those scenes)...but not every 'whoops' is a faceroll, just certain specific ones.

You'll never be able to stay up to date with arena comps and their abbreviations.

Actually you can. There's only so many comps before you've explored every possibility, and then rule out the ones that simply aren't viable, say for instance, triple blood dk. Then you're left with the ones that eventually have names or abbreviations. Plus, most if not all of the ones on that list still exist, or even if they aren't run use the same name. It's sort of become an accepted thing.

Either a combination of first person to come up with the name or first name that catches on. Though people get annoyed with it I actually find the whole 'ludicrous' name thing amusing and somewhat whimsical. I enjoy seeing weird names like Mongolian Earthworm Cleave (that don't seem to make ANY sense), and some names that DO make sense (like Spicy Chicken Cleave), but are still comical.